Cochlear implants (inner ear prostheses) are a means to help profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids, which just apply an amplified and modified sound signal, a cochlear implant is based on direct electrical stimulation of the acoustic nerve. The intention of a cochlear implant is to stimulate nervous structures in the inner ear electrically in such a way that hearing impressions most similar to normal hearing are obtained.
A cochlear prosthesis essentially consists of two parts, the speech processor and the implanted stimulator. The speech processor contains the power supply (batteries) of the overall system and is used to perform signal processing of the acoustic signal to extract the stimulation parameters. The stimulator (implant) generates the stimulation patterns and conducts them to the nervous tissue by means of an electrode array which usually is positioned in the scala tympani in the inner ear. The connection between the speech processor and the implanted receiver can be established by means of encoding digital information in an rf-channel involving an inductively coupled coils system.
Decoding the information within the implant can require envelope detection. Envelope detection of an RF signal within an implant is usually performed with a simple circuit, as shown in FIG. 1, composed of a rectifier diode 4, an RC-network 1 and 2, and a comparator 7. A drawback to this circuit is that the total power consumption of the RC-network, due in part to the ohmic resistor, can be considerable when taking into account the cochlear implant application.
Stimulation strategies employing high-rate pulsatile stimuli in multi-channel electrode arrays have proved to be successful in giving very high levels of speech recognition. One example therefore is the so-called “Continuous Interleaved Sampling (CIS)”—strategy, as described by Wilson B. S., Finley C. C., Lawson D. T., Wolford R. D., Eddington D. K., Rabinowitz W. M., “Better speech recognition with cochlear implants,” Nature, vol. 352:236–238 (1991), which is incorporated herein by reference. For CIS, symmetrical biphasic current pulses are used, which are strictly non-overlapping in time. The rate per channel typically is higher than 800 pulses/sec
Stimulation strategies based on simultaneous activation of electrode currents so far have not shown any advantage as compared to CIS. The basic problem is the spatial channel interaction caused by conductive tissue in the scala tympani between the stimulation electrodes. If two or more stimulation current sources are activated simultaneously, and if there is no correlation between them, the currents will flow between the active electrodes and do not reach the regions of neurons which are intended to be stimulated. The problem might get less severe with new stimulation electrode designs, where the electrodes are much closer to the modiolus as compared to existing electrodes, as described by Kuzma J., “Evaluation of new modiolus-hugging electrode concepts in a transparent model of the cochlea,” proc. 4th European Symp. on Pediatric Cochlear Implantation, 's-Hertogenbosch, The Netherlands (June 1998), which is incorporated herein by reference.
For high-rate pulsatile stimulation strategies, some patient specific parameters have to be determined. This is done some weeks after surgery in a so called “fitting”-procedure. For given phase duration of stimulation pulses and for given stimulation rate, two key parameters have to be determined for each stimulation channel:                1. the minimum amplitude of biphasic current pulses necessary to elicit a hearing sensation (Threshold Level, or THL); and        2. the amplitude resulting in a hearing sensation at a comfortable level (Most Comfort Level, or MCL).        
For stimulation, only amplitudes between MCL and THL (for each channel) are used. The dynamic range between MCL and THL typically is between 6–12 dB. However, the absolute positions of MCLs and THLs vary considerably between patients, and differences can reach up to 40 dB. To cover these absolute variations, the overall dynamic range for stimulation in currently used implants typically is about 60 dB.
At the moment, MCLs and THLs are estimated during the fitting procedure by applying stimulation pulses and asking the patient about his/her subjective impression. This method usually works without problems with postlingually deaf patients. However, problems occur with prelingually or congenitally deaf patients, and in this group all ages—from small children to adults—are concerned. These patients are usually neither able to interpret nor to describe hearing impressions, and only rough estimations of MCLs and THLs based on behavioral methods are possible. Especially the situation of congenitally deaf small children needs to be mentioned here. An adequate acoustic input is extremely important for the infant's speech and hearing development, and this input in many cases can be provided with a properly fitted cochlear implant.
One approach for an objective measurement of MCLs and THLs is based on the measurement of the EAPs (Electrically evoked Action Potentials), as described by Gantz B., Brown C. J., Abbas P. J., “Intraoperative Measures of Electrically Evoked Auditory Nerve Compound Action Potentials,” American Journal of Otology 15 (2):137–144 (1994), which is incorporated herein by reference. In this approach, the overall response of the acoustic nerve to an electrical stimulus is measured very close to the position of nerve excitation. This neural response is caused by the superposition of single neural responses at the outside of the axon membranes. The amplitude of the EAP at the measurement position is between 10 μV and 1000 μV. Information about MCL and THL at a particular electrode position can first of all be expected from the so called “amplitude growth function,” as described by Brown C. J., Abbas P. J., Borland J., Bertschy M. R., “Electrically evoked whole nerve action potentials in Ineraid cochlear implant users: responses to different stimulating electrode configurations and comparison to psychophysical responses,” Journal of Speech and Hearing Research, vol. 39:453–467 (June 1996), which is incorporated herein by reference. This function is the relation between the amplitude of the stimulation pulse and the peak-to-peak voltage of the EAP. Another interesting relation is the so called “recovery function”. Here, stimulation is achieved with two pulses with varying interpulse-interval. The recovery function as the relation of the amplitude of the 2nd EAP and the interpulse-interval allows one to draw conclusions about the refractory properties and particular properties concerning the time resolution of the acoustic nerve.